Wave soldering machines are traditionally used for soldering components onto an electronic circuit (either in the case of components inserted into the circuit or components surface-mounted on this circuit), and also for tinning terminations of electronic components or else for soldering contact strips onto electronics supports such as hybrid circuits.
The design of these machines is thus such that, after they have been fluxed in an upstream zone of the machine (principally so as to deoxidize the metallic surfaces in order to facilitate their subsequent wetting by the soldering material), the components to be soldered or to be tinned are brought into contact with one or more waves of liquid soldering material obtained by pumping the soldering material bath contained in a vat through a nozzle.
Application of the chemical flux at the entry of the machine generally gives rise to the requirement for the user to carry out, at the exit of the machine, after soldering or tinning, an operation of cleaning the products, often with the aid of chlorinated solvents, making it possible to remove the flux residues which remain on the circuit or the component.
Although these machines are traditionally open to the ambient air atmosphere, growing use of an inert atmosphere in these wave soldering machines is in practice observed, principally with the aim of avoiding the formation of oxide layers on the surface of the soldering material bath because of its exposure to air, but also in an international environmental protection context, in combination with the use of low-activity flux leaving a very low amount of residue on the circuits, which makes it possible essentially to eliminate the subsequent operation of cleaning these circuits, and therefore the use of toxic solvents.
For its part, reflow soldering relies on a different principle: it consists in depositing a soldering paste on predetermined regions located on the surface of the circuit, in bringing the components and the soldering paste into contact, then in heating the soldering paste so as locally to produce soldered joints. In the case of reflow soldering, the flux is included in the composition of the soldering paste, which furthermore comprises, in particular, a powder of a metallic alloy (most often a tin-lead alloy).
Here again, increasing use of controlled inert atmosphere is observed, both with a desire to improve the quality of the solder joint obtained and also here again to allow the use of soldering pastes containing low-residue fluxes for which it is possible to avoid the final operation of cleaning the circuits.
In this context, the meniscograph is often a key element of the workshop or assembly site in electronics. It is used in particular for evaluating the solderability of the components (qualification test of the components), the wettability of the substrates used for fabricating the circuits, both in the case of substrates of the printed circuit type or hybrid circuits (qualification of the substrates), the wetting time, the efficiency of the fluxes, the efficiency of the soldering creams, etc.
Its principle, which is well known to the person skilled in the art, is that of determining the wetting angle of a solder on a sample (both in the case of a sample of the component type or of the substrate type), by measuring the wetting force exerted by the solder on the surface of the sample during immersion of this sample in a liquid solder bath. A classical relationship connects, in particular, the resultant force (between the wetting force proper and the buoyancy), the wetting angle, the density of the alloy tested, the liquid/vapour surface tension of the system, and the volume of the sample immersed in the molten alloy.
It is thus classically considered that, for a wetting angle .theta. greater than or equal to zero and less than or equal to 30.degree., the wettability is qualified as being very good, for .theta. strictly greater than 30.degree. and less than or equal to 40.degree., the wettability is qualified as being good, for .theta. greater than 40.degree. and less than or equal to 55.degree., the wettability is acceptable, for an angle .theta. strictly greater than 55.degree. and less than or equal to 70.degree., the wettability is termed low and, finally, for an angle .theta. strictly greater than 70.degree., the wettability is qualified as being poor.
As indicated hereinabove, soldering processes (both in the case, for example, of reflow soldering and wave soldering), have to date been most commonly carried out under ambient air atmosphere. Commercially available meniscographs were therefore perfectly adapted to this situation, having a structure which is open to the surrounding atmosphere.
In the new context of increasing use of controlled inert atmospheres during the soldering processes, the users of these industries are currently showing increasing interest in being able to obtain meniscographs which can operate under protective atmosphere, containing residual concentrations of oxygen which are as low as a few tens of ppm, or even a few ppm of oxygen.
The aim is thus to be able to reproduce the atmosphere conditions produced on a day to day basis in their soldering furnace or machine, including the most severe and restricting conditions of residual oxygen concentrations.
Thus, the requirements most commonly expressed are as follows:
capability of carrying out comparative evaluation tests of the solderability (the two expressions solderability or wettability are both often used equivalently) of the surfaces between a nitrogen atmosphere comprising a determined residual oxygen concentration and a traditional air atmosphere;
qualification of the components and substrates received by the user, under the atmosphere conditions in which they are subsequently treated in the soldering process used.
Fulfilling such desires of users therefore requires controlled (generally inert) atmospheres to be set up in such meniscographs, under conditions making it possible to obtain perfectly controlled residual oxygen concentrations, including very low concentrations.
In this context, it was apparent to the applicants that the solution to be applied to inertness problems posed by users was not only the production of optimized inertness (rapid inertness time, very low residual oxygen concentrations, economically acceptable gas consumption), but also that the atmosphere set up (therefore the corresponding gas injection) does not constitute interference with the measurement, that is to say the high-precision tool constituted by a wetting balance or meniscograph.
Once defined, these two conditions of efficient and rapid inertness and non-interference with the measurement appear relatively incompatible.
In a more general context, the Applicant Company recently proposed, in the French Patent Application filed under No. FR-A-93.15503, a gas injection device for forming a controlled atmosphere in a confined space, which was particularly suitable for setting up a controlled atmosphere in one or more zones of a wave soldering machine, or else in all or part of a continuous furnace used for applications such as soldering or else annealing, tampering, sintering or any other heat treatment.